Field of the Invention
The present invention relates generally to catalysts. More particularly, the present invention relates to Fischer-Tropsch catalysts. Still more specifically, the present invention relates to a novel method and system for activating Fischer-Tropsch catalysts and production therewith of Fischer-Tropsch synthesis products.
Background of the Invention
The Fischer-Tropsch process was developed in the 1920's as a way of producing hydrocarbons from synthesis gas, i.e. a mixture of hydrogen and carbon monoxide, also known as ‘syngas’. Initially, the process was centered on producing gasoline-range hydrocarbons as automotive fuels. More recently however, the Fischer Tropsch process is increasingly utilized as a method for preparing heavier hydrocarbons, such as diesel fuels, and waxy molecules that can subsequently be converted into clean, efficient lubricants.
Fischer-Tropsch synthesis involves the catalytic reductive oligomerization of carbon monoxide in the presence of hydrogen as follows:nCO+2nH2(CH2)n+nH2O,  (1)where n is an integer and (CH2)n represents paraffinic and olefinic hydrocarbons. Generally, the Fischer-Tropsch process converts a mixture of carbon monoxide and molecular hydrogen into a mixture of hydrocarbons, including saturated hydrocarbons and olefins. Oxygenated hydrocarbons, such as alcohols, and some aromatics may also be formed in a Fischer-Tropsch process. More specifically, products of Fischer-Tropsch synthesis can include gaseous, liquid, heavy oil, and wax products, which are typically further upgraded to various fuels, including gasoline, jet, and diesel fuels, and other value-added hydrocarbon products, particularly liquid hydrocarbons.
Fischer Tropsch synthesis utilizes transition metal catalysts. The most common catalysts are based on cobalt and iron. Commercial plants such as those presently operating in South Africa are based on iron-based catalysts, while those currently under consideration in Qatar are based on cobalt-based catalysts. Fischer Tropsch catalyst is usually manufactured in an oxide form for safety as well as economic reasons, because the oxide forms of the catalysts are typically more stable than the activated or reduced form of the catalyst, which is generally pyrophoric. Once transported to a Fischer-Tropsch synthesis site, the catalyst is reduced either in situ in a Fischer-Tropsch synthesis reactor or in a dedicated activation vessel. If reduced outside the synthesis reactor, the catalyst typically is either encapsulated and sent to the reactor or transported to the reactor in the reduced form. Although unreduced catalyst may also be sent to a Fischer-Tropsch synthesis reactor to be reduced during the actual Fischer Tropsch synthesis, an optimal level of catalyst activity is generally not obtained via this method.
Published activation techniques involve reduction in a stream of gaseous hydrogen and/or carbon monoxide. The characteristics of reduction with carbon monoxide or hydrogen are well known to those skilled in the art. Many laboratory studies have been performed utilizing high purity gases in very controlled environments. Commercial plants do not usually operate with such high purity gases due to the potential for contamination and the associated high cost. This creates a challenge when reducing Fischer-Tropsch catalyst. A typical approach has been to utilize a pressure swing adsorption (PSA) system that produces very high purity hydrogen (e.g., 99.5 volume percent to 99.999 volume percent). The contaminants in the PSA product hydrogen are typically carbon monoxide and carbon dioxide, which are generally present at levels in the range of from about 0.1 ppmv to about 10 ppmv. Other contaminants such as hydrocarbon gases (primarily methane) are usually kept in the range of from about 0.1 ppmv to 10 ppmv. A more cost effective means of providing hydrogen is the use of hydrogen separation membranes. However, the maximum purity that can typically be achieved with a hydrogen membrane is around 95 volume percent. Such hydrogen separation membranes have conventionally not been considered due to the potential negative impact of the 5 volume percent non-hydrogen components on the catalyst activation, due to subsequent catalyst poisoning considerations.
Accordingly, there is a need in the art for a new method and system that enable Fischer-Tropsch catalyst activation utilizing activation gas streams comprising primarily hydrogen, and which are produced via relatively economical apparatus for hydrogen separation. Such hydrogen separation apparatus may produce hydrogen having a purity that is less than that generally obtained via pressure swing adsorption, e.g. less than about 99.5 volume percent. Desirably, such a system and method allow activation of Fischer-Tropsch catalysts utilizing hydrogen produced via a hydrogen separation membrane.